/Engine/Plugins/Runtime/MeshModelingToolset/Source/ModelingComponents/Public/Drawing/PolyEditPreviewMesh.h
"Extrude Type" duplicates the input faces, offsets them using FExtrudeMesh, and stitches them together
void InitializeExtrudeType ( const [FDynamicMesh3](API\Runtime\GeometryCore\DynamicMesh\FDynamicMesh3) * SourceMesh, const [TArray](API\Runtime\Core\Containers\TArray)< int32 > & Triangles, const FVector3d & TransformedOffsetDirection, const FTransform3d * MeshTransform, bool bDeleteExtrudeBaseFaces )